1. Field of the Invention
This invention relates to an image-forming member for electrophotography which is used to form images by utilizing electromagnetic wave such as light including for example, ultraviolet ray, visible ray, infrared ray, x ray, gamma ray and the like.
2. Description of the Prior Art
Heretofore, there have been used inorganic photoconductive materials such as Se, CdS, ZnO and the like and organic photoconductive materials such as poly-N-vinylcarbazole, trinitrofluorenone and the like as a photoconductive material for photoconductive layers of electrophotographic image-forming members.
However, they are suffering from various drawbacks. For example, since Se has only a narrow spectral sensitivity range with respect to for example visible light, its spectral sensitivity is widened by incorporation of Te or As. As a result, an image-forming member of Se type containing Te or As is improved in its spectral sensitivity range, but its light fatigue is increased. On account of this, when the same, one original is to be continuously copied repeatedly, the density of the copied images is inadvantageously decreased, and fog occurs in the background of the image, and further undesirable ghost phenomenon takes place.
In addition, Se, As and Te are extremely harmful to man. Therefore, when an image-forming member is prepared, it is necessary to use a specially designed apparatus which can avoid contact between man and those harmful substances. Further, after preparation of an image-forming member having such a photoconductive layer formed of those substances, if the photoconductive layer is partly exposed, part of such layer is scraped off during the cleaning treatment for the image-forming member and mingles with developer, is scattered in copying machine and contaminates copied image, which causes contact between man and the harmful substances.
When Se photoconductive layer is subjected to a continuous and repetitive corona discharge, the electric properties are frequently deteriorated since the surface portion of such layer is crystallized or oxidized.
Se photoconductive layer may be formed in an amorphous state so as to have a high dark resistance, but crystallization of Se occurs at a temperature as low as about 65.degree. C. so that the amorphous Se photoconductive layer is easily crystallized during handling, for example, at ambient temperature or by friction heat generated by rubbing with other members during image forming steps, and the dark resistance is lowered.
On the other hand, as for an electrophotographic image-forming member of binder type using ZnO, CdS and as photoconductive layer-constituting material, formation of the photoconductive layer having the desired properties is difficult because such layer consists of photoconductive material and binder resin and the former must be uniformly dispersed into the latter. Therefore, parameters for determining the electrical and photoconductive, or physical and chemical properties of the photoconductive layer must be carefully controlled in forming the desired photoconductive layer. Accordingly, the image-forming member having such photoconductive layer is not suitable for the mass production.
The binder type photoconductive layer is so porous that it is adversely affected by humidity and its electric properties are deteriorated when used at a high humidity, which results in formation of images having poor quality. Further, developer is allowed to enter into the photoconductive layer because of the porosity, which results in lowering release property and cleaning property. In particular, when the used developer is a liquid developer, the developer penetrates into the photoconductive layer so that the above disadvantages are enhanced.
CdS itself is poisonous to man. Therefore, attention should be paid so as to avoid contact with CdS and dispersion thereof.
ZnO photoconductive layer of binder type has low photosensitivity and narrow spectral sensitivity range and exhibits remarkable light fatigue and slow photoresponse.
Electrophotographic image-forming members comprising an organic photoconductive material such as poly-N-vinylcarbazole, trinitrofluorenone and the like have such drawbacks that the photosensitivity is low and the spectral sensitivity range with respect to for example the visible light region is narrow and in a shorter wave length region.
In order to solve the above mentioned problems, the present inventors have researched amorphous silicon (hereinafter called "a-Si") and succeeded in obtaining an electrophotographic photosensitive member free from these drawbacks.
Since electric and optical properties of a-Si film vary depending upon the manufacturing processes and manufacturing conditions and the reproducibility is very poor (Journal of Electrochemical Society, Vol. 116, No. 1, pp 77-81. January 1969). For example, a-Si film by vacuum evaporation or sputtering contains a lot of defects such as voids so that the electrical and optical properties are adversely affected to a great extent. Therefore, a-Si had not been studied for a long time. However, in 1976 success of producing p-n junction of a-Si was reported (Applied Physics Letters, Vol. 28, No. 2, pp. 105-107, Jan. 15, 1976). Since then, a-Si drew attentions of scientists. In addition, luminescence which can be only weakly observed in crystalline silicon (c-Si) can be observed at a high efficiency in a-Si and therefore, a-Si has been researched for solar cells (for example, U.S. Pat. No. 4,064,521).
However, a-Si developed for solar cells can not be directly used for the purpose of photoconductive layers of practical electrophotographic image-forming members.
Solar cells take out solar energy in a form of electric current and therefore, the a-Si film should have a low dark resistance for the purpose of obtaining efficiently the electric current at a good SN ratio [photo-current (Ip)/dark current (Id)], but if the resistance is so low, the photosensitivity is lowered and the SN ratio is degraded. Therefore, the dark resistance should be 10.sup.5 -10.sup.8 ohm.cm.
However, such degree of dark resistance is so low for photoconductive layers of electrophotographic image-forming members that such a-Si film can not be used for the photoconductive layers.
Photoconductive materials for electrophotographic apparatuses should have gamma value at a low light exposure region of nearly 1 since the incident light is a reflection light from the surface of materials to be copied and power of the light source built in electrophotographic apparatuses is usually limited.
Conventional a-Si can not satisfy the conditions necessary for electrophotographic processes.
Another report concerning a-Si discloses that when the dark resistance is increased, the photosensitivity is lowered. For example, an a-Si film having dark resistance of about 10.sup.10 ohm.cm shows a lowered photoconductive gain (photocurrent per incident photon). Therefore, conventional a-Si film can not be used for electrophotography even from this point of view.
Other various properties and conditions required for photoconductive layers of electrophotographic image-forming member such as electrostatic characteristics, corona ion resistance, solvent resistance, light fatigue resistance, humidity resistance, heat resistance, abrasion resistance, cleaning properties and the like have not been known as for a-Si films at all.
This invention has been accomplished in the light of the foregoing. The present inventors have continued researches and investigations with great zeal concerning application of a-Si to electrophotographic image-forming member. The invention is based on the discovery that lamination of a hydrogenated amorphous silicon (hereinafter called a-Si:H) layer and an organic compound layer described in the following provides an electrophotographic image-forming member which can be used with sufficient practicality and is extremely superior to the conventional image-forming member in almost all respects.